1. Field of the Invention
The present invention relates to a method and an apparatus for treating a substrate by stable discharge under an atmospheric pressure, in particular, an organic substance removing method and apparatus which are effective to removing (ashing) of organic substances and removing of remaining organic substances after washing a glass substrate in an LSI manufacturing process, an active matrix type LCD manufacturing process or the like.
2. Description of the Related Art
In a semiconductor manufacturing process such as a large scale integrated (LSI) manufacturing process and an active matrix type liquid crystal display (LCD) manufacturing process, a photoresist made of a photosensitive resin is used for mask formation. The photoresist is removed normally by ashing after completion of a desired process, because the resist is cured and carbonized and cannot be removed by a peeling solution. Ashing represents that active oxygen molecule, active ozone molecule, active oxygen atom or the like produced by discharge chemically acts on a resist film made of an organic substance, and the organic substance is removed. Ashing is a kind of combustion reaction.
To produce combustion reaction, it is necessary to produce radical (generic name of chemically active molecule, active atom or the like) which produces occurs oxidation reaction. Radical related to oxidation reaction includes oxygen atom/molecule classes such as oxygen molecule, excitation oxygen molecule, oxygen molecule ion, oxygen atom, oxygen atom ion, ozone molecule, excitation ozone molecule, ozone molecule ion or the like. The oxygen atom/molecule classes have energy levels of a ground state and various excitation states. Also, oxygen compound such as oxygen dinitride (N.sub.2 O), carbon monoxide, carbon dioxide has an energy level of a ground state or an excitation state.
Commonly, the higher a reactivity of radical is, the higher a reaction speed is, and the reactivity is high in order of oxygen molecule, ozone molecule and oxygen atom. Therefore, since a throughput gets high in an atmosphere including a number of oxygen atoms, it is advantageous to the ashing. On the other hand, when a number of radicals having high reactivity exist, a damage to a substrate gets larger. As a result, it is necessary to set a suitable process condition in accordance with a substrate to be treated.
Since combustion reaction is a reaction in a surface, the number of radicals and a reaction temperature are important factors in a reaction speed. Commonly, when the reaction temperature (substrate temperature) is high, the reaction speed gets high. Since a number of resists is made of a heat curable resin; when the substrate temperature is too high, a removing speed is reduced because the resist is cured. Therefore, ashing processing is performed commonly at about 200.degree. C.
A method using discharge and a method using ultraviolet radiation are used to produce radical related to oxygen. The method using discharge includes a method for producing glow discharge plasma in a low pressure and a method using corona discharge in an atmospheric pressure.
The method for producing glow discharge plasma in a low pressure is commonly called low pressure glow discharge. In this method, a substrate to be treated is held into a reaction container capable of reducing a pressure. A pressure in the reaction container is controlled by a suitable reactive gas (oxygen, nitrogen oxide, carbon oxide or the like) at constant. Electromagnetic field is applied to the reactive gas by a pair of electrodes or inductive coupling type coils, thereby ionizing the reactive gas. A frequency of the electromagnetic field to be applied is a direct current (DC) to a microwave having about 2.45 GHz and is a wade range. Commonly, electromagnetic energy is applied by a pair of electrodes in a frequency of from the DC to an RF (radio frequency) having about several MHz and applied by the inductive coupling type coils in a frequency equal to and more than the RF. Since the microwave of about 2.45 GHz can be transmitted as a traveling wave in a quartz used as a wall material of a vacuum container, there is a case wherein energy is supplied as an electromagnetic wave without using electrodes in microwave plasma and ionization is generated. In this case, the electromagnetic wave is supplied as a traveling wave, a stationary wave, or a mixture of a traveling wave and a stationary wave. A reactive gas is exhausted from an exhaust apparatus connected to a reaction container and supplied from the reactive gas supply apparatus, simultaneously.
In the method using corona discharge in an atmospheric pressure, a perturbed (non-uniform) electric field is generated in the atmospheric pressure and ozone is mainly produced by corona discharge within the electric field. Since the perturbed electric field is used, this method is not suitable to a uniform processing in principle. A reason to produce ozone is as follows. Mean free pass in collision is short because of an atmospheric pressure. Also, oxygen atom ion or the like having large internal energy collides with neutron particle repeatedly and change with ozone molecule having a metastable state which life time is long.
In the method using ultraviolet radiation, ultraviolet rays are irradiated from a light source (commonly a low pressure mercury light) having wavelengths of no more than 200 nm and 200 to 300 nm to oxygen or dinitrogen monoxide, to produce ozone and excitation ozone. In principle, since ions are not produced by ultraviolet radiation, only ozones are produced. There is a case wherein two-photon absorption occurs in very low probability and ions are produced.
As described above, the method for producing radical related to oxygen includes two types roughly. One type is a method for mainly producing ozones, such as a corona discharge method and an ultraviolet radiation method. The other type is a method for producing various kind of radicals including oxygen atoms and ions, such as a low pressure glow discharge method. Though a damage to a substrate is small in the ozone producing method, a processing time is very short in the radical producing method. Therefore, the low pressure glow discharge method is commonly used as an ashing processing.
It is confirmed that the low pressure glow discharge method has desired processing capability. However, since a reaction space must maintain at a low pressure, various limitations, that is, increase of an apparatus cost by setting an exhaust apparatus for reducing a pressure, increase of a cost by using a reaction container having a sufficiently mechanical strength with respect to a low pressure state, complication of automatic mechanisms due to difficulty of transfer mechanisms under a high vacuum state, increase of an apparatus weight and an apparatus setting area by setting an exhaust apparatus and increase of a reaction container weight, reduction of throughout due to a vacuum exhaust time and a reaction container leak time and the like, are produced.
Since an ion action is utilized, a damage to a substrate cannot be neglected. This is a marked problem with high integration and high performance of a semiconductor. It is considered that a substrate damage by ions produces by formation of ion sheath within plasma. The ion sheath produces in a case wherein an electron temperature and an ion temperature in plasma do not agree with each other and it is in thermally unbalance state. It has been known that an electron temperature in a common low pressure glow discharge is several eV (several ten thousand degree) and an ion temperature is several ten meV (several hundred degree). Commonly, the lower a pressure is, the higher the electron temperature is. In ECR (electron cyclotron resonance) discharge at a low pressure, the electron temperature may be ten and several eV. Therefore, it is considered that a substrate damage by ions increases in accordance with decrease of the pressure. Experimently, the lower the pressure is, the larger the substrate damage is.
When substrate treatment is performed at a low pressure, the above problem produces. On the other hand, a method for treating a substrate by producing ozones at an atmospheric pressure provides advantages, such as simplifying an apparatus, low apparatus cost, and providing a damage free substrate as well as other similar advantages. However, since substrate treatment time is long, practical use is, limited partically. Therefore, it is desired to obtain an ashing apparatus which has not an exhaust apparatus, an apparatus cost is low and a substrate damage is small.